Biaxially stretched polyester film and method for producing the same

ABSTRACT

A biaxially stretched polyester film and a method for producing the same are provided. The biaxially stretched polyester film includes a polyester resin base layer and a matte layer. The polyester resin base layer includes: (1) 50 to 95 wt % of a polyester resin base material, and an intrinsic viscosity of the polyester resin base material being between 0.5 and 0.8 dL/g; and (2) 0.01 to 5 wt % of a high viscosity polyester resin material, and an intrinsic viscosity of the high viscosity polyester resin material being between 0.9 and 1.1 dL/g. The matte layer includes: (1) 50 to 95 wt % of a polyester resin matrix material, and an intrinsic viscosity of the polyester resin matrix material being between 0.5 and 0.8 dL/g; and (2) 0.3 to 40 wt % of a plurality of filler particles, and the filler particles having an average particle size of between 0.15 μm and 10 μm.

CROSS-REFERENCE TO RELATED PATENT APPLICATION

This application claims the benefit of priority to Taiwan PatentApplication No. 108134670, filed on Sep. 25, 2019. The entire content ofthe above identified application is incorporated herein by reference.

Some references, which may include patents, patent applications andvarious publications, may be cited and discussed in the description ofthis disclosure. The citation and/or discussion of such references isprovided merely to clarify the description of the present disclosure andis not an admission that any such reference is “prior art” to thedisclosure described herein. All references cited and discussed in thisspecification are incorporated herein by reference in their entiretiesand to the same extent as if each reference was individuallyincorporated by reference.

FIELD OF THE DISCLOSURE

The present disclosure relates to a polyester film, and moreparticularly to a biaxially stretched polyester film and a method forproducing the same.

BACKGROUND OF THE DISCLOSURE

Compared to the general plastic films produced by the cast film processor the blown film process, the biaxially stretched polyester films (alsocalled biaxially oriented polyester film, BOPET film) produced by thebiaxial stretching process have better performance and widerapplication. Accordingly, the biaxial stretching process is the mainmethod for producing the polyester film. However, in certainapplications, the physicochemical properties of the conventionalbiaxially stretched polyester films still have many issues that shouldbe addressed.

For example, U.S. Pat. No. 3,154,461A discloses a matte-finish polymericfilm prepared from a thermoplastic such as polyethylene terephthalate orpolypropylene. The film includes incompressible particles (i.e., calciumcarbonate particles or silicon oxide particles) having a particle sizeof between 0.3 μm and 20 μm and a concentration of between 1 wt % and 25wt %. However, the film has a degree of mattness that is unsatisfactoryfor many applications thereof.

German Patent No. DE2353347A1 discloses a method for producing a milkypolyester film having one or more layer structures. The method includespreparing a mixture including linear polyester particles, and themixture further includes 3 wt % to 27 wt % of homo-polymer or co-polymerof ethylene or propylene. The method further includes steps of extrudingthe mixture into a film; quenching the film; orienting the film along adirection perpendicular to the film, and then heat setting the film. Thedrawbacks of this method is that the scrap material (essentially amixture of polyester and ethylene copolymer or propylene copolymer)generated during the preparation of the film cannot be recycled for useas a regrind material, and further causes yellowing on the finallyproduced film Therefore, this method is not economical, and the yellowfilm produced with the regrind scrap material is commerciallyunacceptable.

European Patent No. EP0053498B1 discloses a biaxially orientedmultilayer polyester film. The polyester film has a transparent baselayer (B) and an additional layer (A) having a matte appearance appliedto at least one side of the transparent base layer (B). The matteappearance layer (A) consists essentially of a co-polyester havingpolyethylene terephthalate and a plurality of inert inorganic particleshaving an average diameter of between 0.3 μm and 20 μm. Based on thetotal weight of the layer having the matte appearance, the content ofthe inert inorganic particles is between 3 wt % and 40 wt %. The filmhas a high degree of matteness (i.e., a gloss value of less than 15) anda transparency of greater than 60%. However, the film still leaves moreto be desired in some applications or markets.

SUMMARY OF THE DISCLOSURE

In response to the above-referenced technical inadequacies, the presentdisclosure provides a biaxially stretched polyester film and a methodfor producing the same.

In one aspect, the present disclosure provides a biaxially stretchedpolyester film including a polyester resin base layer and a matte layerformed on a side surface of the polyester resin base layer. Thepolyester resin base layer includes: a polyester resin base material anda high viscosity polyester resin material. Based on the total weight ofthe polyester resin base layer, (1) a content of the polyester resinbase material is between 50 wt % and 95 wt %, in which an intrinsicviscosity of the polyester resin base material is between 0.5 dL/g and0.8 dL/g; and (2) a content of the high viscosity polyester resinmaterial is between 0.01 wt % and 5 wt %, in which the high viscositypolyester resin material is dispersed in the polyester resin basematerial, and an intrinsic viscosity of the high viscosity polyesterresin material is between 0.9 dL/g and 1.1 dL/g. The matte layerincludes: a polyester resin matrix material and a plurality of fillerparticles. Based on the total weight of the matte layer, (1) a contentof the polyester resin matrix material is between 50 wt % and 95 wt %,in which an intrinsic viscosity of the polyester resin matrix materialis between 0.5 dL/g and 0.8 dL/g; and (2) a content of the plurality offiller particles is between 0.3 wt % and 40 wt %, in which the pluralityof filler particles are dispersed in the polyester resin matrixmaterial, and the plurality of filler particles have an average particlesize of between 0.15 μm and 10 μm.

In another aspect, the present disclosure provides a method forproducing a biaxially stretched polyester film. The method includes:feeding a polyester resin base material and a high viscosity polyesterresin material into a first extruder; feeding a polyester resin matrixmaterial and a plurality of filler particles into a second extruder;using a co-extrusion method to co-extrude the polyester resin basematerial and the high viscosity polyester resin material that aredisposed in the first extruder, and the polyester resin matrix materialand the plurality of filler particles that are disposed in the secondextruder, so that the polyester resin base material and the highviscosity polyester resin material are collectively formed into apolyester resin base layer, and the polyester resin matrix material andthe plurality of filler particles are collectively formed into a mattelayer that is disposed on a side surface of the polyester resin baselayer; and biaxially stretching an un-stretched polyester film formed bythe polyester resin base layer and the matte layer, so that a biaxiallystretched polyester film is formed. Based on the total weight of thepolyester resin base layer, a content of the polyester resin basematerial is between 50 wt % and 95 wt %, and a content of the highviscosity polyester resin material is between 0.01 wt % and 5 wt %. Anintrinsic viscosity of the polyester resin base material is between 0.5dL/g and 0.8 dL/g, and an intrinsic viscosity of the high viscositypolyester resin material is between 0.9 dL/g and 1.1 dL/g. Based on thetotal weight of the matte layer, a content of the polyester resin matrixmaterial is between 50 wt % and 95 wt %, and a content of the pluralityof filler particles is between 0.3 wt % and 40 wt %. An intrinsicviscosity of the polyester resin matrix material is between 0.5 dL/g and0.8 dL/g, and the plurality of filler particles have an average particlesize of between 0.15 μm and 10 μm.

Therefore, the biaxially stretched polyester film of the presentdisclosure can exhibit a matte effect on its single sided surfacethrough the technical solutions of “a matte layer being formed on a sidesurface of the polyester resin base layer.” and “the polyester resinbase layer includes: a polyester resin base material and a highviscosity polyester resin material; based on the total weight of thepolyester resin base layer, (1) a content of the polyester resin basematerial is between 50 wt % and 95 wt %, in which an intrinsic viscosityof the polyester resin base material is between 0.5 dL/g and 0.8 dL/g;and (2) a content of the high viscosity polyester resin material isbetween 0.01 wt % and 5 wt %, in which the high viscosity polyesterresin material is dispersed in the polyester resin base material, and anintrinsic viscosity of the high viscosity polyester resin material isbetween 0.9 dL/g and 1.1 dL/g.” and “the matte layer includes: apolyester resin matrix material and a plurality of filler particles;based on the total weight of the matte layer, (1) a content of thepolyester resin matrix material is between 50 wt % and 95 wt %, in whichan intrinsic viscosity of the polyester resin matrix material is between0.5 dL/g and 0.8 dL/g; and (2) a content of the plurality of fillerparticles is between 0.3 wt % and 40 wt %, in which the plurality offiller particles are dispersed in the polyester resin matrix material,and the plurality of filler particles have an average particle size ofbetween 0.15 μm and 10 μm.” Moreover, the matte layer can have goodcompatibility with the polyester resin base layer, and the entirebiaxially stretched polyester film can still maintain good transparency.

These and other aspects of the present disclosure will become apparentfrom the following description of the embodiment taken in conjunctionwith the following drawings and their captions, although variations andmodifications therein may be affected without departing from the spiritand scope of the novel concepts of the disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure will become more fully understood from thefollowing detailed description and accompanying drawings.

FIG. 1 is a cross-sectional view of a biaxially stretched polyester filmaccording to an embodiment of the present disclosure.

FIG. 2 is a partial enlarged view of a region II of FIG. 1.

FIG. 3 is a flow chart of a method for producing the biaxially stretchedpolyester film according to the embodiment of the present disclosure.

FIG. 4 is a schematic view of a co-extrusion extruder according to theembodiment of the present disclosure.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

The present disclosure is more particularly described in the followingexamples that are intended as illustrative only since numerousmodifications and variations therein will be apparent to those skilledin the art. Like numbers in the drawings indicate like componentsthroughout the views. As used in the description herein and throughoutthe claims that follow, unless the context clearly dictates otherwise,the meaning of “a”, “an”, and “the” includes plural reference, and themeaning of “in” includes “in” and “on”. Titles or subtitles can be usedherein for the convenience of a reader, which shall have no influence onthe scope of the present disclosure.

The terms used herein generally have their ordinary meanings in the art.In the case of conflict, the present document, including any definitionsgiven herein, will prevail. The same thing can be expressed in more thanone way. Alternative language and synonyms can be used for any term(s)discussed herein, and no special significance is to be placed uponwhether a term is elaborated or discussed herein. A recital of one ormore synonyms does not exclude the use of other synonyms. The use ofexamples anywhere in this specification including examples of any termsis illustrative only, and in no way limits the scope and meaning of thepresent disclosure or of any exemplified term. Likewise, the presentdisclosure is not limited to various embodiments given herein. Numberingterms such as “first”, “second” or “third” can be used to describevarious components, signals or the like, which are for distinguishingone component/signal from another one only, and are not intended to, norshould be construed to impose any substantive limitations on thecomponents, signals or the like.

Biaxially Stretched Polyester Film

Referring to FIG. 1 and FIG. 2, the present embodiment discloses abiaxially stretched polyester film 100 (also called biaxially orientedpolyester film, BOPET film) The biaxially stretched polyester film 100includes a polyester resin base layer 1 and a matte layer 2 formed on aside surface of the polyester resin base layer 1. More specifically, thebiaxially stretched polyester film 100 is formed by the polyester resinbase layer 1 and the matte layer 2, and the biaxially stretchedpolyester film 100 is formed through a co-extrusion process and abiaxial-stretching process sequentially.

By the composition selections and the concentration adjustments of thepolyester resin base layer 1 and the matte layer 2, the biaxiallystretched polyester film 100 can exhibit a matte effect on its singlesided surface. That is, one side surface of the biaxially stretchedpolyester film 100 can exhibit a matte effect, and the entire biaxiallystretched polyester film 100 can still maintain good transparency.

In order to achieve the above objectives, in the present embodiment, thepolyester resin base layer 1 has a thickness T1 ranging from 1 μm to 100μm, and preferably ranging from 10 μm to 100 μm.

In addition, the polyester resin base layer 1 includes: a polyesterresin base material 11 and a high viscosity polyester resin material 12.The high viscosity polyester resin material 12 is dispersed in thepolyester resin base material 11.

In addition, the polyester resin base material 11 is a polymer preparedfrom a condensation reaction between a dibasic acid and a diol or aderivative thereof. That is, the base material of the polyester resinbase layer 1 is mainly a polyester material. Preferably, the polyestermaterial is polyethylene terephthalate (PET) or polyethylene naphthalate(PEN), but the present disclosure is not limited thereto. Accordingly,the polyester resin base layer 1 enables the entire biaxially stretchedpolyester film 100 to have a good transparency, and the polyester resinbase layer 1 can provide good support to the matte layer 2. It should benoted that the polyester resin base layer 1 of the present embodimentpreferably does not include any filler particle (i.e., inorganic fillerparticles).

It is worth mentioning that the above-mentioned raw material dibasicacid that forms the polyester material is at least one selected from thegroup consisting of terephthalic acid, isophthalic acid, 1,5-naphthalenedicarboxylic acid, 2,6-naphthalene dicarboxylic acid 2,6-naphthalenedicarboxylic acid, 1,4-naphthalene dicarboxylic acid, dibenzoic acid,diphenylethane dicarboxylic acid, diphenylphosphonium dicarboxylic acid,indole-2,6-dicarboxylic acid, 1,3-cyclopentanedicarboxylic acid,1,3-cyclohexanedicarboxylic acid, 1,4-cyclohexanedicarboxylic acid,malonic acid, dimethylmalonic acid, succinic acid, diethyl3,3-succinate, glutaric acid, 2,2-dimethylglutaric acid, adipic acid,2-methyladipate, trimethyl adipate, pimelic acid, azelaic acid, azelaicacid, suberic acid, and dodecanedioic acid.

The above-mentioned raw material diol that forms the polyester materialis at least one selected from the group consisting of ethylene glycol,propylene glycol, hexamethylene glycol, neopentyl glycol,1,2-cyclohexanedimethanol, 1,4-cyclohexanedimethanol, 1,10-decanediol,1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, and2,2-bis(4-hydroxyphenyl)propane, and bis(4-hydroxyphenyl)anthracene.

It is worth mentioning that in terms of the thickness of the base layer,if the thickness T1 of the polyester resin base layer 1 is greater thanthe upper limit of the above thickness range, the transparency of thebiaxially stretched polyester film 100 will be deteriorated. Conversely,if the thickness T1 of the polyester resin base layer 1 is less than thelower limit of the above thickness range, the polyester resin base layer1 cannot provide good support to the matte layer 2.

In terms of the content of each components, based on the total weight ofthe polyester resin base layer 1 being 100 wt %, a content of thepolyester resin base material 11 is between 50 wt % and 95 wt %, and acontent of the high viscosity polyester resin material 12 is between0.01 wt % and 5 wt %.

In terms of the physicochemical property of each component, an intrinsicviscosity (abbreviated as IV) of the polyester resin base material 11 isbetween 0.5 dL/g and 0.8 dL/g. That is, the polyester resin basematerial 11 is a polyester resin that has an intermediate viscosity.Different from the polyester resin base material 11, an intrinsicviscosity of the high viscosity polyester resin material 12 is between0.9 dL/g and 1.1 dL/g.

According to the selections of the contents and intrinsic viscosities ofthe polyester resin base material 11 and the high viscosity polyesterresin material 12, the viscosity difference between the polyester resinbase layer 1 and the matte layer 2 can be reduced, thereby reducing theresidual stress of the polyester resin base layer 1 and the matte layer2 when co-extruded, and further improving the compatibility between thepolyester resin base layer 1 and the matte layer 2. Furthermore, theoverall transparency of the biaxially stretched polyester film 100 canbe maintained above a certain level.

However, if the content of the high viscosity polyester resin material12 is greater than the upper limit of the above content range (i.e.,greater than 5 wt %), the haze value of the polyester resin base layer 1will become too high, and the high viscosity polyester resin material 12cannot be uniformly dispersed in the polyester resin base material 11,so that the film formation of the polyester resin base layer 1 may havedefects, and the transparency of the entire biaxially stretchedpolyester film 100 will be deteriorated. Conversely, if the content ofthe high viscosity polyester resin material 12 is less than the lowerlimit of the above content range (i.e., less than 0.01 wt %), the entirebiaxially stretched polyester film 100 will not achieve the desiredphysical and chemical properties (such as haze value).

It is worth mentioning that in the polyester resin base layer 1, thehigh viscosity polyester resin material 12 is also a polymer prepared bya condensation reaction between a dibasic acid and a diol or aderivative thereof. That is, the high viscosity polyester resin material12 is also a polyester material.

In order to achieve the above objectives, in the present embodiment, thematte layer 2 has a thickness T2 ranging from 1 μm to 100 μm, andpreferably ranging from 5 μm to 85 μm.

If the thickness T2 of the matte layer 2 is greater than the upper limitof the above thickness range, the transparency of the biaxiallystretched polyester film 100 will be deteriorated. Conversely, if thethickness T2 of the matte layer 2 is less than the lower limit of theabove thickness range, the haze value of the matte layer 2 will becomeinsufficient, so that the matte layer 2 cannot achieve the desiredphysicochemical properties.

Further, the matte layer 2 includes a polyester resin matrix material 21and a plurality of filler particles 22. The plurality of fillerparticles 22 are uniformly dispersed in the polyester resin matrixmaterial 21.

In terms of the content of each component, based on the total weight ofthe matte layer 2 that is 100 wt %, a content of the polyester resinmatrix material 21 is between 50 wt % and 95 wt %. In addition, acontent of the plurality of filler particles 22 is between 0.3 wt % and40 wt %, and preferably between 0.3 wt % and 25 wt %.

In terms of the physicochemical property of each component, an intrinsicviscosity of the polyester resin matrix material 21 is between 0.5 dL/gand 0.8 dL/g. That is, the polyester resin matrix material 21 is apolyester resin that has an intermediate viscosity.

It is worth mentioning that in the matte layer 2, the polyester resinmatrix material 21 is also a polymer prepared by a condensation reactionbetween a dibasic acid and a diol or a derivative thereof. That is, thepolyester resin matrix material 21 is also a polyester material.

Since in the biaxially stretched polyester film 100 of the presentembodiment, the main materials respectively used in the polyester resinbase layer 1 and the matte layer 2 are both polyester materials, thebiaxially stretched polyester film 100 can have good productivity andprocessability. In addition, the scrap material generated from thebiaxially stretched polyester film 100 during the production process canalso be recycled into the production process as a recycled material, andthe film produced from the recycled material does not have issues suchas yellowing.

Further, in the present embodiment, an average particle size of theplurality of filler particles 22 is between 0.15 μm and 10 μm. Based onthe total weight of the matte layer 2 that is 100 wt %, a content of theplurality of filler particles 22 is between 0.3 wt % and 40 wt %.

Accordingly, when a light ray passes through the matte layer 2, thelight ray can be scattered by the matte layer 2 due to the introductionof the plurality of filler particles 22. The biaxially stretchedpolyester film 100 can exhibit a matte effect on its single sidedsurface, while the entire biaxially stretched polyester film 100 canstill maintain good transparency.

In other words, by introducing the plurality of filler particles 22 intothe matte layer 2 and by the parameter selections of each component thatincludes content range and physicochemical property, the biaxiallystretched polyester film 100 can exhibit a matte effect on its singlesided surface, while the entire biaxially stretched polyester film 100can still maintain good transparency.

In terms of content, if the content of the plurality of filler particles22 is greater than the upper limit of the above content range (i.e.,greater than 40 wt %), the haze value of the matte layer 2 will becometoo high, the plurality of filler particles 22 cannot be uniformlydispersed in the polyester resin matrix material 21, and the pluralityof filler particles 22 may aggregate with each other. Therefore, theoverall transparency of the biaxially stretched polyester film 100 maybe deteriorated. Conversely, if the content of the plurality of fillerparticles 22 is less than the lower limit of the above content range(i.e., less than 0.3 wt %), the haze value of the matte layer 2 willbecome insufficient.

In terms of average particle size, if the average particle size of theplurality of filler particles 22 is greater than the upper limit of theabove average particle size range, the overall transparency of thebiaxially stretched polyester film 100 will be deteriorated. Conversely,if the average particle size of the plurality of filler particles 22 isless than the lower limit of the above average particle size range, thesurface roughness of the matte layer 2 will become too low.

Further, in an embodiment of the present disclosure, the plurality offiller particles 22 can be further divided into a plurality of firstfiller particles 221 and a plurality of second filler particles 222 inaccordance with the difference in average particle size. The pluralityof first filler particles 221 and the plurality of second fillerparticles 222 are mixed with each other, the plurality of first fillerparticles 221 have a first average particle size, and the plurality ofsecond filler particles 222 has a second average particle size.

The first average particle size of the plurality of first fillerparticles 221 is between 0.15 μm and 2 μm, and the second averageparticle size of the plurality of second filler particles 222 is between2 μm and 10 μm. In addition, an absolute value of a difference betweenthe first average particle size of the plurality of first fillerparticles 221 and the second average particle size of the plurality ofsecond filler particles 222 is not less than 1 μm, and preferably notless than 3 μm.

In an embodiment of the present disclosure, the plurality of firstfiller particles 221 and second filler particles 222 are spherical orirregularly shaped silicon dioxide particles, but the present disclosureis not limited thereto. For example, the materials of the first fillerparticles 221 and second filler particles 222 may be respectivelyselected from the group consisting of silicon dioxide, titanium dioxide,cerium dioxide, aluminum hydroxide, magnesium hydroxide, aluminum oxide,magnesium oxide, boron oxide, calcium oxide, calcium carbonate, bariumcarbonate, strontium titanate, barium titanate, calcium titanate,magnesium titanate, calcium sulfate, barium sulfate, lithium phosphate,calcium phosphate, magnesium phosphate, boron nitride, aluminum nitride,carbon black, talc, kaolinite, and crosslinked polymer.

It is also worth mentioning that in an embodiment of the presentdisclosure, the plurality of first filler particles 221 and theplurality of second filler particles 222 are mixed with each other basedon a weight ratio ranging between 1:9 and 9:1, so that a particle sizedistribution curve obtained by a dynamic light scattering method (DLS)of a mixture mixed by the plurality of first filler particles 221 andthe plurality of second filler particles 222 exhibits a single-peakdistribution. Accordingly, the uniformity of the surface roughness ofthe matte layer 2 can be effectively improved, and the biaxiallystretched polyester film 100 can have a low gloss matte surface andachieve a high transparency effect.

According to the above configuration, a side surface 23 of the mattelayer 2 away from the polyester resin base layer 1 (i.e., the outersurface 23 of the matte layer 2) has an average roughness (Ra) ofbetween 150 nm and 950 nm. The entire biaxially stretched polyester film100 of the present embodiment has a transparency (at the visiblewavelength range) of not less than 80%, a gloss value of not more than60% (preferably between 30% and 55%), and a haze value of not less than4% (preferably between 50% and 80%).

Based on the above physicochemical properties, the biaxially stretchedpolyester film 100 of the present embodiment is particularly suitablefor use as a release film, a barrier film, or a special packagingmaterial.

In an embodiment of the present disclosure, when the biaxially stretchedpolyester film 100 is used as the release film, the matte layer 2 of thebiaxially stretched polyester film 100 may be coated with a releaseagent. In other words, the biaxially stretched polyester film 100 mayserve as a substrate for coating various types of resins, such asthermoplastic polyurethane (TPU) and chlorinated polypropylene resin(CPP).

In an embodiment of the present disclosure, when the biaxially stretchedpolyester film 100 is used as the barrier film, the matte layer 2 of thebiaxially stretched polyester film 100 can be vapor-deposited with aninorganic material to improve the effect of blocking moisture and gas.

In an embodiment of the present disclosure, since the biaxiallystretched polyester film 100 has a single-sided matte and hightransparency property, the biaxially stretched polyester film 100 isalso particularly suitable for use as a special package material.

In an embodiment of the present disclosure, the material composition ofthe polyester resin base layer 1 does not include any filler particle,and the material composition of the matte layer 2 does not include anyhigh viscosity polyester resin material. That is to say, the presentdisclosure has excluded the possibility that the material composition ofthe polyester resin base layer 1 includes filler particles, and hasexcluded the possibility that the material composition of the mattelayer 2 includes a high viscosity polyester resin material.

Method for Producing Polyester Film

The related description of the biaxially stretched polyester film 100 isdescribed above, and a method for producing the biaxially stretchedpolyester film 100 will be described below according to an embodiment ofthe present disclosure.

Referring to FIG. 3 and FIG. 4, the present embodiment discloses amethod for producing a biaxially stretched polyester film 100. Themethod for producing the biaxially stretched polyester film 100 includesstep S110, step S120, step S130, and step S140. It should be noted thatthe order of the steps described in the present embodiment and theactual way of operation may be adjusted according to practicalrequirements, and the present disclosure is not limited thereto.

The step S110 includes feeding a polyester resin base material 11′ and ahigh viscosity polyester resin material 12′ into a first extruder E1,and then melting the polyester resin base material 11′ and the highviscosity polyester resin material 12′ at a high temperature of between260° C. and 300° C. Moreover, an intrinsic viscosity of the polyesterresin base material 11′ is between 0.5 dL/g and 0.8 dL/g. That is, thepolyester resin base material 11′ is a polyester resin having anintermediate viscosity. Further, an intrinsic viscosity of the highviscosity polyester resin material 12′ is between 0.9 dL/g and 1.1 dL/g.

The step 120 includes feeding a polyester resin matrix material 21′ anda plurality of filler particles 22′ into a second extruder E2, and thenmelting and mixing the polyester resin matrix material 21′ and theplurality of filler particles 22′ at a high temperature of between 260°C. and 300° C. Moreover, an intrinsic viscosity of the polyester resinmatrix material 21′ is between 0.5 dL/g and 0.8 dL/g. That is, thepolyester resin matrix material 21′ is also a polyester resin having anintermediate viscosity. In addition, the plurality of filler particles22′ have an average particle size of between 0.15 μm and 10 μm.

The step 130 includes using a co-extrusion method to co-extrude thepolyester resin base material 11′ and the high viscosity polyester resinmaterial 12′ that are disposed in the first extruder E1, and thepolyester resin matrix material 21′ and the plurality of fillerparticles 22′ that are disposed in the second extruder E2, and thenrapid cooling the extruded materials via a cooled drum E3 (i.e., a drumcooled to 20° C. to 50° C.), so that the polyester resin base material11′ and the high viscosity polyester resin material 12′ are collectivelyformed into a polyester resin base layer 1, and the polyester resinmatrix material 21′ and the plurality of filler particles 22′ arecollectively formed into a matte layer 2 that is disposed on a sidesurface of the polyester resin base layer 1. Accordingly, the polyesterresin base layer 1 and the matte layer 2 together form an un-stretchedpolyester film 100′.

The step 140 includes biaxially stretching the un-stretched polyesterfilm 100′ formed by the polyester resin base layer 1 and the matte layer2, so that the un-stretched polyester film 100′ is formed into abiaxially stretched polyester film 100 having a double-layer structure.

Further, the biaxial stretching method may be, for example, alongitudinal uniaxial stretching method, a transverse uniaxialstretching method, a vertical axis-horizontal axis sequential biaxialstretching method, or a vertical axis-horizon axis simultaneous biaxialstretching method, and the present disclosure is not limited thereto. Inaddition, the above biaxial stretching method may, for example, preheatthe un-stretched polyester film 100′ at an extension temperature ofbetween 50° C. and 150° C. The biaxial stretching method furtherincludes applying a stretching process of 2.0 times to 5.0 times, andpreferably 3.0 times to 4.0 times, in a width direction (or lateraldirection, TD) of the un-stretched polyester film 100′ according todifferent elongation ratios; and then applying a stretching process of1.0 times to 3.0 times, and preferably 1.0 times to 2.5 times, in alength direction (or longitudinal direction, MD) of the un-stretchedpolyester film 100′.

Based on the total weight of the polyester resin base layer 1, a contentof the polyester resin base material 11′ is between 50 wt % and 95 wt %,and a content of the high viscosity polyester resin material 12′ isbetween 0.01 wt % and 5 wt %. In addition, based on the total weight ofthe matte layer 2, a content of the polyester resin matrix material 21′is between 50 wt % and 95 wt %, and a content of the plurality of fillerparticles 22′ is between 0.3 wt % and 40 wt %.

According to the above method for producing the biaxially stretchedpolyester film 100, a side surface 23 of the matte layer 2 away from thepolyester resin base layer 1 has an average roughness (Ra) of between150 nm and 950 nm, and the entire biaxially stretched polyester film 100has a transparency of not less than 80%, a gloss value of not more than60% (preferably between 20% and 40%), and a haze value of not less than4%.

Experimental Results

Hereinafter, exemplary examples 1 to 4 and comparative examples 1 to 2will now be described in detail. However, the exemplary examples areonly used to help in the understanding of the present disclosure, andthe scope of the present disclosure is not limited to these examples.

The biaxially stretched polyester films of the exemplary examples 1 to 4and the comparative examples 1 to 2 can be produced by referring to theabove steps S110 to S140. Among them, the content of each component andthe process parameters are shown in Table 1 below.

Further, the biaxially stretched polyester films of the exemplaryexamples 1 to 4 and the comparative examples 1 to 2 were subjected todifferent tests to obtain different physicochemical properties, such asthe surface roughness of the polyester film, and the transparency, glossvalue, and haze value of the entire polyester film. The relevant testmethods are described below, and the relevant test results are shown inTable 1 below.

The roughness test includes: using a three-dimensional surface roughnessmeter (Otaru Research Institute, trade name SURF CORDER SE-3CK) to testthe center line average roughness (Ra) and the ten-point averageroughness (Rz) of the surfaces of the biaxially stretched polyesterfilms of the exemplary examples 1 to 4 and the comparative examples 1 to2 according to JIS B0601 and JIS B0651 under the conditions of stylustip R2ym, scanning pitch 2μπι, scanning length 1 mm, scanning strips100, cut-off value 0.25 mm and magnification of 5,000 times. Theroughness listed in Table 1 is the center line average roughness (Ra).

The transparency/haze test includes: using a haze meter (TC-HIIIproduced by Tokyo Denshoku Co., Ltd.) to test the transparency (lighttransmittance rate) and the haze value of the biaxially stretchedpolyester films of the exemplary examples 1 to 4 and the comparativeexamples 1 to 2 according to JIS K7705.

The gloss test includes: using a gloss meter (VGS-SENSOR produced byNippon Denshoku Industries Co., Ltd.) to test the gloss value (G60) ofthe biaxially stretched polyester films of the exemplary examples 1 to 4and the comparative examples 1 to 2 according to JIS Z 8741. The testconditions were as follows: the incident angle and the acceptance anglewere both 60° (N=5), and the average value was used.

Table 1 shows the material parameters and test results of the exemplaryand comparative examples.

exemplary exemplary exemplary exemplary terms example 1 example 2example 3 example 4 matte layer thickness of matte layer (μm) 10 80 2080 content of polyester resin 98.2 98.8 80.0 98.8 matrix material (wt %)intrinsic viscosity of 0.6 0.6 0.8 0.6 polyester resin matrix material(dL/g) total content of 1.8 1.2 20.0 1.2 filler particles (wt %) contentof 1.6 1.0 2.0 1.0 first filler particles (wt %) average particle sizeof 2.5 6.0 5.0 6.0 first filler particles (μm) content of 0.2 0.2 18.00.2 second filler particles (wt %) average particle size of 1.4 1.4 2.01.4 second filler particles (μm) polyester resin thickness of polyesterresin 12 100 80 20 base layer base layer (μm) content of polyester resin99.8 99.8 99.8 99.5 base material (wt %) intrinsic viscosity of 0.6 0.60.8 0.6 polyester resin base material (dL/g) content of high viscosity0.01 0.01 0.01 0.50 polyester resin material (wt %) intrinsic viscosityof 0.9 0.9 1.1 1.1 high viscosity polyester resin material (dL/g)content of — — — — filler particles (wt %) average particle size of — —— — filler particles (μm) test results surface roughness (Ra) of 226 520803 652 matte surface (nm) surface roughness (Ra) of 171 321 262 267flat surface (nm) ratio of Ra values of matte 1.32 1.62 3.06 2.44surface to flat surface transparency of 85.1 82.9 83.2 86.5 entirepolyester film (%) gloss value of 37 52 55 59 matte surface (%) glossvalue of 66 64 71 68 flat surface (%) ratio of gloss values of 0.56 0.810.77 0.86 matte surface to flat surface haze value of 60.01 73.80 72.8275.21 entire polyester film (%) terms comparative example 1 comparativeexample 2 matte layer thickness of matte layer (μm) — — content ofpolyester resin — — matrix material (wt %) intrinsic viscosity of — —polyester resin matrix material (dL/g) total content of — — fillerparticles (wt %) content of — — first filler particles (wt %) averageparticle size of — — first filler particles (μm) content of — — secondfiller particles (wt %) average particle size of — — second fillerparticles (μm) resin base thickness of polyester resin 12 100 base layer(μm) content of polyester resin 100 100 base material (wt %) intrinsicviscosity of 0.6 0.6 polyester resin base material (dL/g) content ofhigh viscosity — — polyester resin material (wt %) intrinsic viscosityof — — high viscosity polyester resin material (dL/g) content of 0.3 0.2filler particles (wt %) average particle size of 1.4 1.4 fillerparticles (μm) test results surface roughness (Ra) of 18 24 mattesurface (nm) surface roughness (Ra) of 18 24 flat surface (nm) ratio ofRa values of matte 1 1 surface to flat surface transparency of 89.0 88.4entire polyester film (%) gloss value of 165 167 matte surface (%) glossvalue of 165 167 flat surface (%) ratio of gloss values of 1 1 mattesurface to flat surface haze value of 3.62 4.62 entire polyester film(%)

[Discussion of Test Results]

According to the material and process parameters of the exemplary andcomparative examples as shown in Table 1, the polyester films havingone-sided matte surface of the exemplary examples 1 to 4 all have atransparency of not less than 80% and a matte surface gloss value of notmore than 60%. Further, when the thickness of the matte layer isrelatively thick, the matte layer can have a lower filler particlecontent, which still achieves the same effect. For example, thethickness of the matte layer of the exemplary example 3 is 20 μm, andthe total content of the filler particles is 20 wt %. Comparatively, thethickness of the matte layer of the exemplary example 4 is 80 μm, andthe total content of the filler particles is 1.2 wt % (i.e., a lowerfiller particle content).

Further, in the polyester films of the exemplary examples 1 to 4, aratio of the surface roughness of the matte surface to the surfaceroughness of the flat surface is not less than 1.3 and is substantiallybetween 1.3 and 3.5; and a ratio of the gloss value of the matte surfaceto the gloss value of the flat surface is not more than 0.9 and issubstantially between 0.5 and 0.9. It should be noted that the above“matte surface” refers to the outer surface of the matte layer, and theabove “flat surface” refers to the outer surface of the polyester resinbase layer.

The polyester films of the comparative examples 1 to 2 do not have anymatte layer. The filler particles are added to the polyester resin baselayer of the polyester films, but the content of the filler particles isless than 0.3 wt %. Although the transparency of the polyester films ofthe comparative examples is more than 80%, the gloss value and the hazevalue are both low; therefore, the comparative examples cannot achievethe matte effect of the exemplary examples.

Advantageous Effect

In conclusion, the biaxially stretched polyester film 100 of the presentdisclosure can exhibit a matte effect on its single sided surfacethrough the technical solutions of “a matte layer 2 being formed on aside surface of the polyester resin base layer 1.” and “the polyesterresin base layer 1 includes: a polyester resin base material 11 and ahigh viscosity polyester resin material 12; based on the total weight ofthe polyester resin base layer 1, (1) a content of the polyester resinbase material 11 is between 50 wt % and 95 wt %, in which an intrinsicviscosity of the polyester resin base material 11 is between 0.5 dL/gand 0.8 dL/g; and (2) a content of the high viscosity polyester resinmaterial 12 is between 0.01 wt % and 5 wt %, in which the high viscositypolyester resin material 12 is dispersed in the polyester resin basematerial 11, and an intrinsic viscosity of the high viscosity polyesterresin material 12 is between 0.9 dL/g and 1.1 dL/g.” and “the mattelayer 2 includes: a polyester resin matrix material 21 and a pluralityof filler particles 22; based on the total weight of the matte layer 2,(1) a content of the polyester resin matrix material 21 is between 50 wt% and 95 wt %, in which an intrinsic viscosity of the polyester resinmatrix material 21 is between 0.5 dL/g and 0.8 dL/g; and (2) a contentof the plurality of filler particles 22 is between 0.3 wt % and 40 wt %,in which the plurality of filler particles 22 are dispersed in thepolyester resin matrix material 21, and the plurality of fillerparticles 22 have an average particle size of between 0.15 μm and 10μm.” Moreover, the matte layer 2 can have good compatibility with thepolyester resin base layer 1, and the entire biaxially stretchedpolyester film 100 can still maintain good transparency.

According to the selections of the contents and intrinsic viscosities ofthe polyester resin base material 11 and the high viscosity polyesterresin material 12, the viscosity difference between the polyester resinbase layer 1 and the matte layer 2 can be reduced, thereby reducing theresidual stress of the polyester resin base layer 1 and the matte layer2 when co-extruded, and further improving the compatibility between thepolyester resin base layer 1 and the matte layer 2. Furthermore, theoverall transparency of the biaxially stretched polyester film 100 canalso be maintained above a certain level.

Further, since in the biaxially stretched polyester film 100 of thepresent embodiment, the matrix materials respectively used in thepolyester resin base layer 1 and the matte layer 2 are both polyestermaterials, the biaxially stretched polyester film 100 can have goodproductivity and processability. In addition, the scrap materialgenerated from the biaxially stretched polyester film 100 during theproduction process can also be recycled into the production process as arecycled material, and the film produced from the recycled material doesnot have issues such as yellowing.

The foregoing description of the exemplary embodiments of the disclosurehas been presented only for the purposes of illustration and descriptionand is not intended to be exhaustive or to limit the disclosure to theprecise forms disclosed. Many modifications and variations are possiblein light of the above teaching.

The embodiments were chosen and described in order to explain theprinciples of the disclosure and their practical application so as toenable others skilled in the art to utilize the disclosure and variousembodiments and with various modifications as are suited to theparticular use contemplated. Alternative embodiments will becomeapparent to those skilled in the art to which the present disclosurepertains without departing from its spirit and scope.

What is claimed is:
 1. A biaxially stretched polyester film, comprising:a polyester resin base layer; wherein the polyester resin base layerincludes: a polyester resin base material and a high viscosity polyesterresin material, and based on the total weight of the polyester resinbase layer being 100% wt, (1) a content of the polyester resin basematerial is between 50 wt % and 95 wt %, wherein an intrinsic viscosityof the polyester resin base material is between 0.5 dL/g and 0.8 dL/g;and (2) a content of the high viscosity polyester resin material isbetween 0.01 wt % and 5 wt %, wherein the high viscosity polyester resinmaterial is dispersed in the polyester resin base material, and anintrinsic viscosity of the high viscosity polyester resin material isbetween 0.9 dL/g and 1.1 dL/g; and a matte layer being formed on a sidesurface of the polyester resin base layer; wherein the matte layerincludes: a polyester resin matrix material and a plurality of fillerparticles, and based on the total weight of the matte layer, (1) acontent of the polyester resin matrix material is between 50 wt % and 95wt %, wherein an intrinsic viscosity of the polyester resin matrixmaterial is between 0.5 dL/g and 0.8 dL/g; and (2) a content of theplurality of filler particles is between 0.3 wt % and 40 wt %, whereinthe plurality of filler particles are dispersed in the polyester resinmatrix material, and the plurality of filler particles have an averageparticle size of between 0.15 μm and 10 μm.
 2. The biaxially stretchedpolyester film according to claim 1, wherein the polyester resin baselayer does not include any filler particle, and the matte layer does notinclude any high viscosity polyester resin material.
 3. The biaxiallystretched polyester film according to claim 1, wherein the polyesterresin base layer has a thickness of between 1 μm and 100 μm, and thematte layer has a thickness of between 1 μm and 100 μm.
 4. The biaxiallystretched polyester film according to claim 1, wherein the plurality offiller particles are further divided into: a plurality of first fillerparticles and a plurality of second filler particles; wherein theplurality of first filler particles have a first average particle size,the plurality of second filler particles are mixed with the plurality offirst filler particles, and the plurality of second filler particleshave a second average particle size; wherein the first average particlesize is between 0.15 μm and 2 μm, and the second average particle sizeis between 2 μm and 10 μm.
 5. The biaxially stretched polyester filmaccording to claim 4, wherein an absolute value of a difference betweenthe first average particle size and the second average particle size isnot less than 1 μm.
 6. The biaxially stretched polyester film accordingto claim 4, wherein a side surface of the matte layer away from thepolyester resin base layer has an average roughness (Ra) of between 150nm and 950 nm; wherein the entire biaxially stretched polyester film hasa transparency of not less than 80%, a gloss value of not more than 60%,and a haze value of not less than 4%.
 7. The biaxially stretchedpolyester film according to claim 4, wherein the polyester resin basematerial, the polyester resin matrix material, and the high viscositypolyester resin material are all polymers prepared by a condensationreaction between a dibasic acid and a diol; wherein the materials of theplurality of first and second filler particles are respectively selectedfrom the group consisting of silicon dioxide, titanium dioxide, ceriumdioxide, aluminum hydroxide, magnesium hydroxide, aluminum oxide,magnesium oxide, boron oxide, calcium oxide, calcium carbonate, bariumcarbonate, strontium titanate, barium titanate, calcium titanate,magnesium titanate, calcium sulfate, barium sulfate, lithium phosphate,calcium phosphate, magnesium phosphate, boron nitride, aluminum nitride,carbon black, talc, kaolinite, and crosslinked polymer.
 8. The biaxiallystretched polyester film according to claim 1, wherein an outer surfaceof the matte layer is defined as a matte surface, and an outer surfaceof the polyester resin base layer is defined as a flat surface; whereina ratio of the surface roughness of the matte surface to the surfaceroughness of the flat surface is not less than 1.3, and a ratio of thegloss value of the matte surface to the gloss value of the flat surfaceis not greater than 0.9.
 9. A method for producing a biaxially stretchedpolyester film, comprising: feeding a polyester resin base material anda high viscosity polyester resin material into a first extruder; feedinga polyester resin matrix material and a plurality of filler particlesinto a second extruder; using a co-extrusion method to co-extrude thepolyester resin base material and the high viscosity polyester resinmaterial that are disposed in the first extruder, and the polyesterresin matrix material and the plurality of filler particles that aredisposed in the second extruder, so that the polyester resin basematerial and the high viscosity polyester resin material arecollectively formed into a polyester resin base layer, and the polyesterresin matrix material and the plurality of filler particles arecollectively formed into a matte layer that is disposed on a sidesurface of the polyester resin base layer; and biaxially stretching anun-stretched polyester film formed by the polyester resin base layer andthe matte layer, so that a biaxially stretched polyester film is formed;wherein based on the total weight of the polyester resin base layer, acontent of the polyester resin base material is between 50 wt % and 95wt %, and a content of the high viscosity polyester resin material isbetween 0.01 wt % and 5 wt %; wherein an intrinsic viscosity of thepolyester resin base material is between 0.5 dL/g and 0.8 dL/g, and anintrinsic viscosity of the high viscosity polyester resin material isbetween 0.9 dL/g and 1.1 dL/g; wherein based on the total weight of thematte layer, a content of the polyester resin matrix material is between50 wt % and 95 wt %, and a content of the plurality of filler particlesis between 0.3 wt % and 40 wt %; wherein an intrinsic viscosity of thepolyester resin matrix material is between 0.5 dL/g and 0.8 dL/g, andthe plurality of filler particles have an average particle size ofbetween 0.15 μm and 10 μm.
 10. The method for producing the biaxiallystretched polyester film according to claim 9, wherein the plurality offiller particles are further divided into: a plurality of first fillerparticles and a plurality of second filler particles; wherein theplurality of first filler particles have a first average particle size,the plurality of second filler particles are mixed with the plurality offirst filler particles, and the plurality of second filler particleshave a second average particle size; wherein the first average particlesize is between 0.15 μm and 2 μm, the second average particle size isbetween 2 μm and 10 μm, and an absolute value of a difference betweenthe first average particle size and the second average particle size isnot less than 1 μm.
 11. The method for producing the biaxially stretchedpolyester film according to claim 10, wherein the plurality of firstfiller particles and the plurality of second filler particles are mixedwith each other based on a weight ratio ranging between 1:9 and 9:1, sothat a particle size distribution curve obtained by a dynamic lightscattering method (DLS) of a mixture mixed by the plurality of firstfiller particles and the plurality of second filler particles exhibits asingle-peak distribution.
 12. The method for producing the biaxiallystretched polyester film according to claim 10, wherein a side surfaceof the matte layer away from the polyester resin base layer has anaverage roughness (Ra) of between 150 nm and 950 nm; wherein the entirebiaxially stretched polyester film has a transparency of not less than80%, a gloss value of not more than 60%, and a haze value of not lessthan 4%.